1. Field of the Invention
The present invention is broadly concerned with rapidly hydratable, reversibly swellable modified starch products having unique properties, methods of preparing such starch products and thickener compositions and food products containing the starch products. More particularly, the modified starch products of the invention are in the form of individual, chemically cross-linked starch granules which are capable of undergoing multiple swelling/drying cycles with very little generation of starch solubles during such cycling. The products can be prepared by simultaneously swelling and cross-linking native starch granules in an aqueous system without complete gelatinization thereof, followed by heating the cross-linked granules in excess water. The starch products of the invention can be used to good effect in a variety of food products.
2. Description of the Prior Art
Granular cold water swelling starches are well known. These starches can be prepared by suspending wet native starch granules in rapidly moving hot air of high, followed by low, humidity (U.S. Pat. No. 4,280,851), or by heating starch in an excess of water/alcohol with subsequent removal of liquid (U.S. Pat. No. 4,465,704).
When known granular cold water swelling starches are placed in hot or cold water, the granules swell excessively and release starch solubles into the aqueous phase. Upon drying, the individual swollen starch granules collapse and fuse together. Fused dried granules can be reground, but do not thereafter thicken efficiently and give dull appearing pastes.
As a consequence of these properties, typical cold water swelling starches have only limited utility in food systems where gelling is to be avoided, e.g., in broths or other watery foods. In such systems, the conventional starches swell and gelatinize and release amylose, and upon storage give the food an unappealling mouth feel. In addition, the fact that the known starches are not reversibly swellable (i.e., they are incapable of undergoing successive swelling/drying cycles) limits the utility of the starches.
Another factor important in food grade starches relates to the in vivo digestive properties thereof.
In 1987 Englyst and Cummings at the MRC Dunn Clinical Nutrition Center in Cambridge, UK, proposed a classification of starch based on its likely digestive properties in vivo. They also devised in vitro assay methods to mimic the various digestive properties of starch. Three classes of dietary starch were proposed:
(1) Rapidly Digestible Starch (RDS). RDS is likely to be rapidly digested in the human small intestine; examples include freshly cooked rice and potato, and some instant breakfast cereals. PA1 (2) Slowly Digestible Starch (SDS). SDS is likely to be slowly yet completely digested in the small intestine; examples include raw cereal starch and cooked pasta. PA1 (3) Resistant Starch (RS). RS is likely to resist digestion in the small intestine. RS is thus defined as the sum of starch and starch degradation products not likely to be absorbed in the small intestine of healthy individuals. RS can be subdivided into four categories depending on the cause of resistance (Englyst et al Eur. J. Clin. Nutr. 46(suppl 2):S33, 1992; Eerlingen et al Cereal Chem. 70:339, 1993). PA1 RS.sub.1. Physically inaccessible starch due to entrapment of granules within a protein matrix or within a plant cell wall, such as in partially milled grain or legumes after cooling. PA1 RS.sub.2. Raw starch granules, such as those from potato or green banana, that resist digestion by .alpha.-amylase, possibly because those granules lack micropores through their surface. PA1 RS.sub.3. Retrograded amylose formed by heat/moisture treatment of starch or starch foods, such as occurs in cooked/cooled potato and corn flake. PA1 RS.sub.4. Chemically modified starches, such as acetylated, hydroxypropylated, or cross-linked starches that resist digestion by alpha-amylase. Those modified starches would be detected by the in vitro assay of RS. However, some RS.sub.4 may not be fermented in the colon.
RS.sub.1, RS.sub.2, RS.sub.3 are physically modified forms of starch and become accessible to .alpha.-amylase digestion upon solubilization in sodium hydroxide or dimethyl sulfoxide. RS.sub.4 that is chemically substituted remains resistant to .alpha.-amylase digestion even if dissolved. RS.sub.4 produced by cross-linking would resist dissolution.
RS.sub.3 has been of increasing interest as a food ingredient. Unlike common dietary fiber sources, RS.sub.3 does not hold much water and, thus may be a preferred fiber source for use in low moisture products such as cookies and crackers. Also, RS.sub.3 is free of a gritty mouthfeel, and unlike traditional fiber sources does not significantly alter flavor and textural properties of foods. Those characteristics can improve the processing and quality of foods such as baked and extruded products when RS is added. Furthermore, RS.sub.3 constitutes dietary fiber, and may be assigned much reduced calories.